1. FIELD OF THE INVENTION
This invention relates to an electrode for a fuel cell, and, more particularly, it is concerned with an improved structure of such electrode.
2. DISCUSSION OF BACKGROUND
FIG. 4 of the accompanying drawing indicates a cross-section of a conventional electrode for a fuel cell, in which a reference numeral 1 designates an electrode base, a numeral 2 refers to a catalyst layer, 3 denotes a catalyst penetrated layer, and 4 a matrix layer.
In the following, the method of manufacturing the electrode will be explained. The electrode base 1 should have electrical conductivity and gas permeability, and is obtained by immersing such porous and electrically conductive sheet like carbon paper in a dispersion liquid of polytetrafluoroethylene (hereinafter simply referred to as "PTFE") or by applying such dispersion liquid of PTFE onto such sheet to render it water-repellent.
The catalyst layer 2 is formed, for example, by applying on the above-mentioned electrode base 1 catalyst paste containing therein catalyst powder obtained by having fine particles of platinum carried on the surface of carbon powder and PTFE as a binder, which is heat-treated to a temperature of, for example, 300.degree. C. to 350.degree. C. The matrix layer 4 is formed, for example, by applying on the above-mentioned catalyst layer 2 matrix paste containing therein silicon carbide powder and PTFE as a binding agent, which is heat-treated to a temperature of, for example, 300.degree. C. to 350.degree. C. The catalyst layer 2 and the matrix layer 4 may be heat-treated either separately or jointly.
The above-mentioned catalyst paste and matrix paste may be applied by the spraying method, the curtain coating method, the doctor blade method, and so on. By the application of the above-mentioned catalyst paste on the electrode base 1, there is formed the catalyst penetrated layer 3 owing to penetration of the catalyst paste into the electrode base 1, the depth of which varies depending on various conditions such as diameter of the pores in the electrode base 1, intensity of the water-repellent property, viscosity of the catalyst paste, and so forth. It can therefore be said that depth of the catalyst penetrated layer 3 tends to be readily affected by production lots of the electrode base 1 and the catalyst paste. Further, if the pore diameter and the intensity of water-repellent property are not uniformly distributed in one and the same electrode base 1, the depth of the catalyst penetrated layer 3 becomes non-uniform in its distribution. In addition, PTFE used for treating the electrode base 1 to have the required water-repellent property becomes molten at the time of heat-treatment of the electrode base at a temperature of from 300.degree. C. to 350.degree. C. after application of the catalyst paste, which causes the pore structure of the catalyst layer 2 to change.
Since the conventional electrode for the fuel cell is constructed as mentioned above, there has been a point of problem such that the catalyst paste penetrates into the interior of the electrode base 1 and the pore of the electrode base 1 are filled with the catalyst powder and the binding agent, on account of which the electrode base 1 has a reduced gas diffusibility. In the case of an electrode such as the electrode with rib, wherein an electrolyte is held in the interior of the electrode base 1, there has been a point of problem such that the volume of pores decreases to lower the sustaining quantity of the electrolyte, hence shortened service life of the cell. Also, since the catalyst penetrated into the electrode base 1 is not effectively utilized in the cell reaction, there has been a point of problem such that the utility of the catalyst is low. Further, when the electrode base 1 subjected to the water-repellent treatment, there has been a point of problem such that the electrolyte like phosphoric acid, etc. can be impregnated by application only from the surface side of the matrix layer. Furthermore, the conventional electrode for the fuel cell has its disadvantage such that the catalyst layer 2 and the matrix layer 3 have poor adhesive property, which readily brings about peeling and cracking of the matrix layer 3.